The northwest-dipping Carthage-Colton shear zone, located near the eastern margin of the Grenville province in northern New York, separates the Adirondack Lowlands of the Metasedimentary Belt from the Adirondack Highlands of the Granulite Terrane. Published U/Pb sphene ages are 1156-1103 Ma in the Lowlands and 1050-982 Ma in the Highlands, indicating an ϳ100 m.y. offset in metamorphic ages across the boundary. Our reported hornblende 40 Ar-39 Ar ages are ϳ1060 Ma in the Lowlands and ϳ950 Ma in the Highlands. This confirms the offsets in sphene ages across the Carthage-Colton shear zone and indicates an average cooling rate in both the Lowlands and the Highlands of 1؇-2؇/ m.y. over ϳ100 m.y. Biotite 40 Ar-39 Ar ages from this study yield no apparent age difference, thus approximating the time of final Carthage-Colton shear-zone extension. These 40 Ar-39 Ar data suggest that extensional motion along the Carthage-Colton shear zone occurred between 950 and 920 Ma, postdating the latest recorded compressional activity between 1060 and 1030 Ma and orogenic collapse between 1045 and 1030 Ma in the Metasedimentary Belt. Extensional motion occurring at least 100 m.y. after the last compressional event is probably not related to orogenic collapse, but

A B S T R A C TThe Adirondack Mountains expose two distinct tectonic elements of the Proterozoic Grenville Province of northeastern North America: the Adirondack Lowlands and Highlands. The Lowlands are located along the eastern edge of the Metasedimentary Belt, and the Highlands form the western portion of the Granulite Terrane. The two are separated by the Carthage-Colton Shear Zone (CCSZ). U/Pb titanite and 40 Ar hornblende ages in the Lowlands are ca. 100 m.yr. older than in the Highlands, across the CCSZ. While both the Lowlands and the Highlands record a history of metamorphism during the Elzevirian Orogeny (ca. 1150 Ma), only the Highlands record evidence of a major phase of Grenvillian thermotectonic activity (Ottawan Orogeny) at ca. 1090-1030 Ma. Proposed tectonic models require that the Lowlands were either laterally separated from the Highlands or were at structurally higher levels during this metamorphism. New U/Pb and Ar hornblende ages at 1000 Ma within the shear zone. Thermobarometric constraints indicate that deformation occurred under granulite-facies conditions. Therefore, titanite and hornblende ages are interpreted as cooling ages from a ca. 1050-1030-Ma event. An early transpressive shearing event at ca. 1040 Ma, combined with structural data in the Dana Hill metagabbro that indicate strike-slip displacement along the CCSZ, suggests significant lateral displacement between the Lowlands and the Highlands during the ca. 1090-1030-Ma Ottawan Orogeny, prior to later extension along the CCSZ.

The Elzevir Terrane of the Grenville Orogen in southern Ontario contains metapelites and abundant graphitic marbles that were regionally metamorphosed from the upper greenschist to upper amphibolite facies. Comparative thermometry was undertaken with widely used calibrations for the systems garnet-biotite, calcitedolomite, and calcite-graphite. Temperatures that are obtained from matrix biotites paired with prograde garnet near-rim analyses are usually consistent with those determined using calcite-graphite thermometry. However, calcite-graphite thermometry occasionally yields low temperatures due to lack of equilibration of anomalously light graphite. Application of calcitegraphite and garnet-biotite systems may yield temperatures up to 70°C higher than calcite-dolomite in amphibolite facies rocks. Calcite-dolomite temperatures most closely approach those from calcite-graphite and garnet-biotite when the samples contain a single generation of dolomite and calcite grains contain no visible dolomite exsolution lamellae. However, some of these samples yield temperatures considerably lower than temperatures calculated from calcite-graphite and garnet-biotite thermometry, indicating that the calcite-dolomite thermometer may have been partially reset during retrogression. Estimated peak metamorphic temperatures of regional metamorphism between Madoc (upper greenschist facies) and Bancroft (upper amphibolite facies) range from 500 to 650°C. These results place the chlorite-staurolite isograd at 540°C, the kyanite-sillimanite isograd at 590°C, and the sillimanite-K-feldspar isograd at 650°C. Although each thermometer may have an absolute uncertainty of as much as 50°C, the 50 to 60°C temperature dierences between the isograds are probably accurate to 10 to 20°C. An incomplete picture of the thermal gradients can result from the application of only one thermometer in a given area. Simultaneous application of several systems allows one to recognize and overcome the inherent limitations of each thermometer.

[1] A comprehensive geochronologic database describes a period of late extension across the Metasedimentary Belt (MB) of the Grenville Province in northeastern North America. Because extension continued through much of the young unroofing history of the region, these data do not bracket the timing of final extension across all shear zones and do not constrain the timing of final juxtaposition of terranes. Analysis of K-feldspar in the MB by the 40 Ar-39 Ar method can give information on the very latest perturbations in the area since K-feldspars have multiple, low closure temperatures, ranging from about 150°to 300°C. These multiple diffusion domains require temperature-time modeling to fully interpret argon release spectra. This modeling requires precise knowledge of the temperature of degassing of each step, which usually requires the use of a resistance furnace for sample analysis. We establish a first-order relationship between laser power and temperature, which can be used for multidiffusion domain modeling of K-feldspars. Comparison of samples analyzed by both methods reveals virtually no difference between the systems, supporting the validity of K-feldspar analyses by laser step heating. Our data suggest that using the laser for K-feldspars can give results that are geologically reasonable, precise, and easier to collect. In this case, these data are then applied to the cooling history of the North American Grenville Province. Our K-feldspar analyses show that the latest extensional motion along shear zones in the MB is after 900 Ma, and the region is uplifting as a uniform block by 780 Ma.